Control means for x-ray apparatus



June 1962 R. F. BRYER ETAL 3,038,998

CONTROL MEANS FOR X-RAY APPARATUS Filed Dec. 10, 1959 Exposure Square Pulse SeHing General-or- G are d CW. R. F. Amplifier oscillaror High Insulcmon High Insulation RF Fansformer RFTrAnsFormer Peak Peak RechFier Rectifier Pulse Amplifier Pulse Amplifier And C Mhode And Cathode FoHower Follower Posihve Negative H.T. H.T.

Generator Generaror Anode Cathode SeIecPor Selecl'or swil'ch Swi %ch 1 V INVEN FITTORMEYS 3,038,998 CONTROL MEANS FOR X-RAY APPARATUS Robert Frank Bryer, Northwood, and Donovan Cohin Chadney, Wembley, England, assignors to The General Electric Company Limited, London, Engiand Filed Dec. 10, 1959, Ser. No. 858,692 Claims priority, application Great Britain Dec. 19, 1958 3 Claims. .(Cl. 250-4102) This invention relates to X-ray apparatus, and more particularly to means for controlling the supply to the X-ray tube thereof.

For short exposure work it is more eliective to control the high. tension circuit of the X-ray tube, since pulse switching in the primary circuit of the supply to the tube is not only complicated because of the relatively heavy currents which flow in this circuit, but the minimum duration of exposure is affected by the frequency of the supply; and in the case of a SO-cycle supply the minimum practicable exposure is usually one-hundredth of a second. Difficulties increase as the period of the desired exposure becomes smaller. Even in the case of circuits which are switched on the high tension side difliculties arise when exposures of extremely short duration are required, and with existing circuits it is by no means certain that precise control is maintained over the energisation of the X-ray tube.

It is becoming increasingly desirable to control pulse energisation of X-ray tubes for periods down to only a fraction of a milli-second duration, more especially for diagnostic work in connection with cardiac diseases, and it is an object of the present invention to provide control means which are eminently suited for adaptation to such work.

In X-ray apparatus of the kind in which an X-ray tube is operated by switching its high tension circuit, according to the present invention, the said high tension circuit is arranged to be of substantially constant impedance on load, whereby the X-ray tube may be operated at substantially constant voltage, even for extremely short exposures. Preferably switching is arranged to be effected by means of grid-controlled electronic switch tubes.

When the X-ray apparatus is of the kind which is adapted for switching the X-ray tube on both anode and cathode high tension circuits, it will be preferable to arrange that both circuits are of substantially constant impedance on load.

One form of X-ray apparatus in accordance with the invention will now be described by way of example with reference to the accompanying drawing, which indicates diagrammatically the essential characteristics of a circuit for operating an X-ray tube at varying exposures down to a fraction of a milli-second.

In the drawing, the X-ray tube 1 is shown to be energised from two high'voltage generators 2 and 3, in the anode and cathode circuits respectively. Switching of these circuits is effected by means of switch tubes 4 and 5 which are hard-valve triodes and whose grids are normal- 1y biassed to render the tubes substantially non-conductmg.

The generators 2 and *3, which may be of any convenient form will be found to have equivalent resistive and inductive components of value R L and R L respectively. The generators are therefore each shunted by a resistance of value equal to R (or R and a capacitor of capacitance C (or C in series, and, by arrangement, the values of C and C are such that L =C R and L =C R By this means, the anode and cathode circuits of the X-ray tube become of constant impedance on load.

Thus, when the biassing voltages on the grids of the aten switch tubes 4 and 5 are controlled so that the tubes operate for an exposure, the resultant HT. voltage across the X-ray tube is immediately equivalent to the steady load voltage, with the result that the voltage across the X-ray tube is substantially constant during the whole of the exposure, no matter how short the exposure time. In contrast, if known practice were followed, and there were no adaptation to constant impedance networks, the time constants of the circuits would cause an exponential change of voltage across the X-ray tube, which, for a short exposure time, could result in a change of voltage during the'whole of the exposure time.

Diodes 5, 7 are provided as indicated in the drawing to ensure that never more than half the generator voltage is applied to either of the switch tubes after cut-off.

The remaining parts of the apparatus which are indicated in the drawing are those associated with the grid control circuits of the switch tubes. The means for producing pulse operation of the switch tubes may be of any convenient form, but we prefer to adopt a pulsed radio frequency signal type of control. Since the components and circuits for the various units in the control circuits will be readily ascertainable to suit particular re quirements, the circuits are indicated only as block diagrams.

It is observed that a pulse signal for the exposure, which signal comprises a pulse of radio frequency cycles, of frequency, say, 250 kc./s., is set by means of a common low tension circuit which is coupled to the grid control circuits of the switch tubes in both anode and cathode circuits of the Xray tube, through high insulation radio frequency transformers. For obvious reasons however the output of such transformers is limited and will not suflice to provide a rectified voltage adequate to control switch tubes of the kind required; it is necessary therefore to amplify this signal.

If a common cathode amplifier is used, then inversion of the input signal occurs and the rectifier will be connected to give a negative voltage. However, if a common anode amplifier is used, no such inversion occurs, and the rectifier will be connected to give a positive voltage. Whichever amplifier is used at this stage, final amplification will preferably be provided 'by a cathode follower type of amplifier, the characteristics of which will be determined by the requirements for control of the switch tube.

Since a cathode follower type of amplifier yields a sharp cut-off at the termination of a pulse signal, the duration of an exposure may be carefully controlled. In combination with the voltage control resulting from the use of constant impedance circuits in accordance with the present invention, therefore, it is ensured that each energisation of the X-ray tube may be maintained under strict control irrespective of the time of exposure, and even for exposure periods of only a fraction of a milli-second.

We claim:

1. A circuit for operating an X-ray tulae comprising a high tension source of direct current having an impedance including an inductive component, at least one connection from said high tension source for the X-ray tube, a grid-controlled switch tube in said connection, means for applying to the control grid of said switch tube a first control potential at which the switch tube is conducting and a second control potential at which the switch tube is substantially non-conducting, and a circuit branch in parallel with said high tension source having an impedance including a capacitive component and further having both a resistive component of impedance and a time constant substantially matching respectively the resistive component and the time constant of said high tension source.

2. A circuit for operating an X-ray tube according to claim 1 in which the parallel circuit branch comprises a capacitor and a resistor in series.

3. A circuit for operating an X-ray tube comprising a positive high tension source of direct current and a negative high tension source of direct current, each source having an impedance including an inductive component, a connection from said positive source for the anode of said X-ray tube, a connection from said negative source for the cathode of said X-ray tube, a grid-controlled switch tube in each of said connections, means for applying to the control grid of each said switch tube a first control potential at which the said switch tube is conducting and a second control potential at which the said switch tube is substantially non-conducting to the current from its respective source, and a circuit branch in parallel with each said high tension source, each said circuit branch having an impedance including a capacitive component and further having both a resistive component of impedance and a time constant substantially matching the resistive component and the time constant of its associated high tension source.

References Cited in the file of this patent UNITED STATES PATENTS 2,659,016 Quittner Nov. 10, 1953 2,730,628 Quittner Jan. 10, 1956 2,790,912 Graves et al. Apr. 30, 1957 2,969,464 Chisholm et a1. Jan. 24, 1961 

